A steam turbine converts the kinetic or thermal energy of pressurized steam into useful, mechanical energy. Generally, steam is created in a steam generator or boiler, then passes through stop and control valves into nozzles, which drive a rotor assembly. The rotor assembly may then in turn drive a generator to produce electrical energy. Stop valves and control valves control the operation of a steam turbine by controlling the flow of steam into the nozzles. A control valve typically controls steam entry into the nozzles during normal operation levels. A stop valve is typically held open during normal operation and closed when immediate shut-down is necessary. In some applications, the control valve and stop valve are integrated into a single unit.
At various operating levels, flow characteristics in and around the valve assemblies of a steam turbine may cause instabilities that may lead to vibration. Excessive vibration is undesirable and may lead to component fatigue and premature failure of the valve assemblies. It is known that valve assemblies in use during operation exhibit vibration characteristics arising from vibration of the valve assemblies themselves, as well as from the flow in and around the valve assemblies, often referred to by structural vibration and acoustic vibration, respectively. The valve assemblies may be impacted by the individual vibration effects, as well as the interaction between the structural and acoustic frequencies and modal shapes.
The current solution to correct vibration frequency interaction is to design valve assemblies that are predicted to achieve sufficient separation between their structural and acoustical characteristics. However, as the number of applications in which valve assemblies may be used increases, their operating conditions become less predictable. Further, the demand on valve assemblies is increasing, causing them to operate in harsher, more varied conditions. Accordingly, providing adequate separation between structural and acoustical vibration characteristics is becoming increasingly more difficult, thereby preventing full awareness of the stresses placed on the valve assemblies and, therefore, the fatigue experienced.
There is a desire, therefore, to better understand the vibration characteristics of valve assemblies used in steam turbines. Further, it is preferable to gain an understanding of the vibration characteristics of the valve assemblies during various operational states for use in comparison with vibration characteristics of valve assemblies while in use.